DEET concentration – A new study suggests mosquitoes can learn to associate DEET’s scent with a “reward,” potentially making the repellent less effective—or even attracting bites—when skin concentrations drop but the smell remains.
A person can do everything “right” with DEET and still face a problem—if the repellent wears off unevenly.
Researchers report that mosquitoes can learn to link DEET’s smell with feeding, raising the prospect that under real-world conditions—especially when the concentration fades over time—DEET could become less of a shield and, in certain situations, more of a signal.
The study, published in the Journal of Experimental Biology, was a collaboration between Clément Vinauger, an associate professor at Virginia Tech University, and Claudio Lazzari of the University of Tours in France.
“Mosquitoes are smart enough to seek out the smell,” the study’s findings suggest, but only after repeated exposure changes how they respond.
The central warning comes from one hard contradiction: DEET is designed to repel mosquitoes, yet repeated contact with the scent could teach some insects to keep coming.
“If mosquitoes are repeatedly exposed to DEET, it becomes less effective as a repellent,” said study co-author Claudio Lazzari of the University of Tours, France, in a statement. “This raises concerns that in certain situations, the repellent may even begin to attract some biting insects.”
The researchers describe how that association could form when the chemical’s concentration drops enough that it no longer prevents feeding, while still remaining strong enough for mosquitoes to smell it.
“If a mosquito bites someone who applied DEET to their skin several hours earlier and the concentration of the repellent is too low to repel the mosquito, but still strong enough for the insect to smell it, the mosquito may be more likely to bite people who smell of DEET,” Lazzari said.
Clément Vinauger offered a similar scenario about how the reward can be redefined.
“If mosquitoes can learn to associate the scent of DEET with the prospect of dinner. this could make people wearing DEET more attractive to biting insects under the right conditions. ” he said in a news release. “If someone applies DEET and the concentration fades over time. but a mosquito still manages to feed. the insect may begin associating that smell with a reward. That’s a possibility we should take seriously when we think about how repellents are used in the real world.”.
The experiments centered on the yellow fever mosquito, a species that spreads dengue fever, Zika, yellow fever, and chikungunya—diseases that infect tens of millions of people each year. Overall, mosquitoes kill as many as one million people each year, far more than any other animal.
To test learning, the researchers restrained mosquitoes behind fabric mesh, with a bag of warm blood positioned just out of reach. Once mosquitoes began feeding, researchers introduced the smell of DEET.
After repeating the experiment four times, more than 60% of the insects tried to feed when presented with only the smell of DEET.
Then came a choice test. Mosquitoes were given two human hands—one untreated and one coated with DEET at normal concentrations. Untrained mosquitoes avoided the DEET-treated hand, while trained mosquitoes were drawn to it.
The team also explored whether the “reward” could be something other than blood.
They found mosquitoes could form the same association when sugar, instead of blood, was used as the reward.
Vinauger said the work challenges a long-held assumption about how repellents work.
“The common assumption has always been that repellents work because of their chemistry — that DEET simply smells bad to mosquitoes and they flee or that its chemistry prevents mosquitoes from smelling us. ” he said. “But what we are showing is that the mosquito’s brain can rewrite that response based on experience. What the insect has learned matters just as much as what the chemical does. That, I think, is a paradigm shift.”.
Even with that unsettling possibility, the findings are not an argument for abandoning DEET.
Vinauger said people should not stop using it, calling DEET one of the most effective repellents available—particularly in regions where mosquito-borne disease is common.
“If you’re in tropical regions where disease risk is real, you should use it,” he said in a statement.
What changes is the emphasis on timing and concentration.
The research suggests that protection may depend not just on whether DEET is applied, but on whether it remains active enough to prevent feeding when mosquitoes take their shot.
Vinauger said people may need to reapply regularly so it stays active and provides continuous protection, instead of applying “a lot at once.” He also pointed out that treated clothing may present challenges because DEET concentrations in fabric decline over time.
DEET mosquito repellent yellow fever mosquito dengue Zika chikungunya yellow fever Virginia Tech University of Tours insect learning Journal of Experimental Biology